There is a touch of pathos about the mournful expression and the cumbersome and archaic design of its body, yet when the turtle stirs from the reef with a measured sweep of its fore flippers it inspires only admiration for the grace of its movement.
Seeing a turtle in the Caribbean or the Red Sea has always been a highlight of the dive for me, but encounters such as these have been rare and fleeting until a recent visit to Sipadan, a tiny island off the east coast of Borneo.
At Sipadan, turtle sightings are so commonplace that they are taken for granted by divers after their first day, so enthusing about turtles immediately marks you out as a new arrival.
Throughout the year turtles can be seen on nearly every dive, sometimes every few yards, and since turtles spend much of their time in the shallows they are easy to snorkel with too. The lazy observer can sit on the end of the jetty watching turtles swim by just below the surface, occasionally surfacing to breathe.
Mating takes place at the surface, often an ungainly procedure involving much rolling about and waving of flippers as the amorous male tries to mount the female. A pair have even been momentarily mistaken for a diver in distress! Of the seven species of sea turtles in the world, at least three occur at Sipadan: the abundant green turtle, the hawksbill and an occasional Pacific or olive ridley. Most turtles are carnivores or omnivores, though the mainly vegetarian green turtle prefers a diet of sea grass.
Hawksbills have a prominent beak, enabling them to scrape the reef face for sponges and other encrusting invertebrates, and to prise out molluscs and crack them open. They resemble green turtles, but the horny scutes or shields covering the bony shell overlap in hawksbills, other than in extremely young and old animals, and they are smaller on average than green turtles, whose carapace can be one metre long.
Pacific ridleys are smaller still and have more scutes (6-9 on either side of the carapace) than hawksbills or green turtles, both of which have four costal pairs. Turtles rest on the reef top or on ledges beneath coral overhangs, though they can often be seen cruising along the reef edge.
Large remoras may attach themselves to turtles and act as cleaners, removing various external parasites.
The female turtle comes ashore to lay her eggs and laboriously hauls her heavy body up the beach at night, using her fore flippers, which leave tracks like tyre marks in the sand, varying according to species. She picks a site above the high water line and uses her fore flippers to dig a shallow pit big enough for her body, then digs an egg chamber in this with her hind flippers. When the eggs have been laid, she flings sand over them and fills in the pit before returning to the sea. The whole process may take several hours and exhausts her.
Turtles are very sensitive to disturbance until after they have started to lay their eggs, and should not be approached at this stage. Once they have committed themselves to egg-laying they pay scant attention to anything else and can be viewed if proper care is taken.
The green turtle nests on Sipadan, and has many other nesting grounds in sites as far flung as Turkey and Costa Rica, in Central America. Research there has provided useful information about the movements of turtles but much of the turtle's life at sea, particularly the first year after hatching, remains a mystery.
The Pacific coast of Costa Rica is renowned for an astonishing phenomenon known as an arribada, involving Pacific ridleys. Here, egg laying is synchronised at intervals of two to four weeks, mainly during July to December. Up to 120,000 turtles may come ashore in the space of a few nights, with the majority on one particular night. Late arrivals frequently excavate earlier nests while digging their own.
Since incubation takes nearly 8 weeks, the next arribada happens well before the previous eggs have hatched. Many eggs are destroyed by this unfortunate timing, and predators (mainly coyotes, raccoons, opossums and coatis in Costa Rica) account for further losses.
Newly hatched turtles are vigorous and energetic, but only a few survive the initial scramble down the beach and out over the reef without being snatched up and eaten by birds or crabs on land and by fish in the water. They usually leave the nest en masse at dusk, when they stand the best chance of survival, being least visible.
During my stay on Sipadan a nest hatched in broad daylight but the hatchlings were rescued by a warden who released them after sunset.
Having survived all the hazards of infancy to reach maturity (this may take from 15 to 50 years for a green turtle) the adult animal has the advantage of size against most predators, but can fall victim to sharks, and I have seen turtles with a missing back flipper, probably due to shark attack.
The biggest threat to turtles comes from commercial exploitation. Not only are eggs taken for food, but adults are taken for their meat and shells and this frequently involves extremely cruel practices.
Despite the large numbers of certain species in some areas the balance of their existence is delicate: even large populations can be vulnerable, and without sufficient protection other populations are dwindling.